Sub-100A range line width pattern fabrication

ABSTRACT

Sub-100A line width patterns are formed on a member by electron beam conversion and fixing of a resist that arrives at the reaction zone point by surface migration into a resist pattern of a precise thickness and width while the member rests on an electron backscattering control support.

This is a division, of application Ser. No. 011,360 filed Feb. 12, 1979,now U.S. Pat. No. 4,197,332, which is a continuation-in-part ofapplication Ser. No. 845,527, filed Oct. 26, 1977 now abandoned.

DESCRIPTION Technical Field

As it becomes useful to increase the number of devices included in anindividual part of an apparatus, the physical size of each device willbecome smaller. This trend has been progressing rapidly in theelectronic apparatus field such as integrated circuits and magneticarrays. Through techniques, such as electron beam fabrication, thesmallest exploratory devices that have been made are 500 A to 100 A.Smaller dimensions have not been achievable because of electronscattering effects. The technique of lithography, that is, the formationof a pattern in a resist layer which is later used to protect regions tobe differentiated from other regions of a broad area, has produced thenarrowest line width patterns that have appeared thus far in the art. Anelectron beam has been used to alter the properties of a layer of aresist material so that the region where the electron beam strikes theresist can be separated from the remainder of the resist for furtherprocessing.

However, as the art developed, and the physical size of the lines in thepattern became smaller, limits began to be encountered even in the fineresolution provided by the electron beam, in that, too much of theresist in an ordinary coating was being irradiated. Through the use of afinely focused electron beam, a resist condensed onto the sample surfacefrom the vapor was polymerized providing a finer line width whichthrough ion beam etching produced a finer pattern. This work was setforth in Microelectronics and Reliability, Vol. 4, pp. 103-104, 1965,Pergamon Press. The vapor resist technique produced lines on the orderof 500 A. In further development of the art, the effects of electronbackscattering from the substrate was also found to be affecting andlimiting the resolution of the irradiated resist pattern. Anon-backscattering substrate was developed and is described in U.S. Pat.No. 3,971,860. This technology of U.S. Pat. No. 3,971,860 permittedpattern line widths as fine as 300 A.

It is particularly desirable and a substantial breakthrough in devicefabrication will be provided when the dimensions become less than 300 Aand particularly less than 100 A. In this range of sizes, the dimensionof the device for many materials is approaching both the wavelength orscattering length of electrons in conventional semiconductors and thedistances involved in such physical processes as quantum mechanicaltunnelling, and hence, entirely new devices will be available in theart.

Best Mode For Carrying Out The Invention

The present invention allows dimensions below 300 A and into the sub-100A range to be obtained. Patterns with line widths of less than 100 A arefabricated by electron beam converting and fixing a pattern of resisthaving precise thickness and width dimensions on a thin film type memberthat is supported by an electron backscattering control substrate. Theresist is present on the surface of the member in a thickness that isless than the required thickness for the desired pattern and arrives atthe point of impact of the electron beam reaction zone by surfacemigration. The electron beam converts and fixes the resist on thesurface and through surface migration, the pattern builds to the properdimensions. Where it is found to be desirable, the surface migratingresist species may be replenished from a vapor should it become depletedduring electron beam writing.

Once the resist pattern is formed with the proper dimensions, it may beused, as is, in such applications as selective contacting and lightmodulation, or, the resist can be employed through an etching operationsuch as ion etching, to separate a pattern in the thin film type member.The pattern in the thin film type member can in turn then be used insitu or transferred by replication with X-rays for device fabrication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph of a metallic 600 A line patternillustrating the resolution achievable at the present state of the art.

FIG. 2 is an electron micrograph of an 80 A metallic line patternproduced in accordance with the invention.

FIG. 3 is a diagram illustrating the scattering effect by electron beamirradiation on a conventional resist layer and substrate.

FIG. 4 is a diagram illustrating the resist pattern formation inaccordance with the invention.

FIG. 5 is a schematic diagram showing the general arrangement of theelectron beam fabrication apparatus employed in the invention.

FIG. 6 is a chart of steps employed in fabricating the metallic patternlines in accordance with the invention.

FIG. 7 is an electron micrograph of resist indicating variations inthickness.

DISCLOSURE OF INVENTION

In efforts to provide resolution in pattern line widths in the less than100 A range it is apparent that, in such a scale, not only is thephysical size of importance but also the raggedness of the edges of thelines becomes significant. This may be seen in comparing FIG. 1 labelled"Prior Art" and FIG. 2 wherein the variations in the edge of the 600 Astate of the art line of FIG. 1 are greater than the total 80 A linewidth in FIG. 2. The scales are indicated in the figure.

The technique of the invention involves the use of an electron beamwhich converts and fixes resist material on the surface of a thin filmtype member supported by a backscattering control substrate to a precisethickness and width. The precision is achieved by electron scatteringcontrol both from the substrate and from the resist itself.

Referring to FIG. 3 a diagram labelled "Prior Art" is provided. In thediagram the resist 1 is present in a coating with a thicknessapproaching the ultimate desired thickness dimension. The electron beam2 is focused on the surface in a small dimension but the irradiatedvolume due to electron scattering both in the heavy resist layer and inthe substrate produce the irradiated volume 3 in the resist which ismuch larger than the focused electron beam 2.

Referring next to FIG. 4, in accordance with the invention a diagram isprovided which illustrates the performance of the resist layer which isthinner than the ultimate required thickness. In FIG. 4 the resist 5builds upon the surface of the thin film member 4. The resist 5 isconverted and fixed to exposed material 6 in the reaction zone at thepoint of impact of the focused electron beam 2 and further resist 5arrives by surface migration until the required size of convertedmaterial 6 is formed. The growth of the converted resist 6 under thebeam 2 is primarily vertical with the width progressing at a lower rate.The precision is achieved directly in thickness and indirectly in width.A backscattering control substrate 7 is provided which permits electronsto pass through and escape, and together with only growing as muchirradiated resist as required, electron scattering is controlled.

It should be noted that FIG. 4 is merely a schematic explanation ofobserved results set forth to aid in understanding and should be viewedwith an awareness of the limitations in measurement at these sizes.

In accordance with the invention, a resist pattern 6 in the vicinity ofabout 300 A thick formed on a thin film type member 4 supported so thatelectrons do not scatter back into the resist from the substrate 7 willproduce patterns with sub-100 A line widths.

Referring next to FIG. 5 there is shown a functional schematic diagramof an apparatus for practicing the invention. The apparatus involves afocused electron beam 2 from a source 8 having the output thereofcontrolled by a monitor 9 through a communication medium functionallyshown as a cable 10. A thin film type member 11, which is preferably ametal layer is positioned on a non-electron backscattering substrate 12which in turn is resting on a support grid 13, all positioned such thata resist 5 present in a thickness less than the required dimension forthe pattern to be formed grows in the reaction zone at the point ofimpact of the electron beam 2 to a greater precisely selectablethickness and width 6. The duration of the electron beam 2 and hence theheight and width of the resist pattern 6 is controlled by monitoring themember of electrons that are scattered. In FIG. 5 this is illustrated bymonitoring the amount of the electron beam 2 that passes throughaperture 14 and reaches the monitor 9. Other ways of monitoring resistbuildup are detecting forward or backward scattered electrons andsecondary electrons. With the apparatus of FIG. 5, the electron beam 2converts and fixes just enough resist to provide the desired pattern 6in thickness. As the resist pattern increases in height it becomes widerand this effect can be correlated with line width. The thin substrate 12permits the electrons to pass through and not be scattered back toprovide uncontrolled resist irradiation.

The electron beam 2 is focused so that in the absence of resist itsubstantially passes through the aperture 14 to the monitor 9. When theresist 6 builds to the desired height and hence width, the grown resistitself produces electron scattering which causes less electrons to passthrough the aperture 14. This is illustrated by dashed lines in FIG. 5.On a scale of 100% electrons sensed before resist growth, when themonitor 9 senses, for example 80%, then 20% of the electrons are beingscattered by the grown resist and the grown resist 6 will now be thickenough and wide enough. The amount of charge of electrons monitored willvary with the materials involved and one skilled in the art can readilyestablish an appropriate variation. At this point the monitor 9communicates to the source 8 through the communication medium 10 to stopthe electron beam 2. Should it be desirable to replenish the resist asit surface migrates to the reaction zone at the point of impact of theelectron beam, this is preferably done by providing an environmentalsource such as a vapor 15.

The sequence of steps involved in the technique of the invention isillustrated in connection with FIG. 6 wherein a flowchart sets forth thesteps involved. The following discussions of the steps are correlatedwith the apparatus of FIG. 5.

Referring to Step 1, a thin film type member 11, preferably a metallayer, on which the desired pattern is ultimately to be formed isprovided on a suitable backscattering control substrate 12. Thebackscattering control substrate 12 is strong enough to support themetal 11, is thin enough and is supported at 13 sufficiently far awaysuch that scattered electrons escape and are not reflected back toprovide undesired irradiation of the resist. A carbon film may be usedas the substrate 12 that is from 10 A to 1000 A thick having a supportgrid 13 with openings that are 125 microns ±125 microns. Some othersubstrate materials that are suitable are Si, Si₃ N₄, SiO₂, Al₂ O₃, andpolyimide and collodion.

Substrates of 60 nm thick Si₃ N₄ membranes are particularlyadvantageous. The substrates may be prepared as follows. <100> oriented,2 ohm-cm, N type silicon wafers 200 μm thick are thermally oxidized onboth sides to a thickness of 500 nm by the well known dry-wet-drychemical process. After masking the back rough surface of the wafer witha conventional photoresist such as that known in the trade as AX1350J,the oxide is removed from the front smooth surface with a solution of9:1 buffered HF which has an etch rate of 63 nm/min. About 100 nm of Si₃N₄ is then chemically vapor deposited on the front surface of the waferat 810° C. using SiH₄ and NH₃ as the reactant gases. A pattern is thenetched into the SiO₂ on the back rough surface of the wafer usingbuffered HF and conventional photoresist techniques. The Si₃ N₄ does nothave to be masked because only about 20 nm is removed during the HFetch.

The wafers are then put into buffered HF for 10 seconds to remove thenative oxide just before using an anisotropic silicon etching techniqueto produce the silicon truncated pyramid support structure of element isin FIG. 5. Anisotropic etching is accomplished in a mixture of 40 g ofpyrocatechol, 250 cc of ethylenediamine, and 80 cc of water, at themixture boiling point of 118° C. in a water cooled reflux apparatus. The200 μm thick silicon wafer is etched through the SiO₂ mask completely tothe Si₃ N₄. Only 20 nm of Si₃ N₄ will be removed during the process at arate of 8.5 nm/hr since SiO₂ etches at 17 nm/hr. It may be seen thatsilicon is advantageous for the support structure 13 because of thelarge difference in etch rates between the <100> direction in silicon(50 μm/hr), and the <110> direction (30 μm/hr) and <111> directions (3μm/hr). Substrate 12 membranes up to 1 micron thick are acceptable forsubstrate use. However, for ultra high resolution structure, membranesless than 150 nm thick are typically used. The substrate 12 when mountedwith an adjacent 200 μm thick Si support 13 can be spin-coated with aresist in a vacuum chuck without breakage.

It is essential only that the resist 5 be converted by the electron beam2 and that the conversion in width and thickness be controllable andthat the grown resist 6 is of precise dimensions. The pattern 6 may beused as is, or it may serve in a subsequent processing step as a basisto delineate a final pattern from the rest of the film type layer 11.The resist 5 must have the properties of conversion and fixing in thepresence of an electron beam and the ability to migrate across thesurface. A satisfactory resist material is the class of materials knownin the art as contamination resists. One contamination type of resist isthe vapor present from the silicone oil usually employed in oildiffusion pumps. This is presumed to be a silicone vapor. Many organicmaterials when applied as in a thin enough layer 5 can be altered by anelectron beam and thereby converted and fixed and hence are suitable forthis type of resist formation. The mechanism by which the electron beamconverts the material into a resist is not fully understood at thistime. The thickness dimension less than the ultimate pattern thicknessmay be achieved, for example, from the vapor or by spinning on a diluteresist liquid and is in the vicinity of 100 nanometers.

In the case of the class of materials known as contamination resists,two sources are useable: background oil vapors present in the vacuumsystem and intentionally deposited vacuum pump oils. Background oilvapors are useable to prepare as small as 8 nm wide and 10 nm thick Pd₄₀Au₆₀ lines. The resist 5 is formed from a vapor of oil in the system on10 nm thick carbon substrate 12 membrane. A perspective of the carerequired may be gained from the fact that although the electron beamdiameter is less than 1 nm, lines in the vicinity of 8 nm wide areobtained.

In order to enable one skilled in the art to select an appropriatecontamination resist 5 the following tables are provided. TABLE 1describes the resist 5 formation of four typical diffusion pump oilscorrelated with the electron exposure dose on two types of substrate 12.

                  TABLE 1                                                         ______________________________________                                        Oil           Electron exposure dose (C/cm)                                   measured      on Si     on Pd.sub.40 Au.sub.60                                ______________________________________                                        Convoil-20    4.8 × 10.sup.-5                                                                   7.4 × 10.sup.-6                                 DC704         7.0 × 10.sup.-6                                                                   8.7 × 10.sup.-7                                 DC705         6.1 × 10.sup.-7                                                                   5.9 × 10.sup.-7                                 Santovac-5    6.3 × 10.sup.-7                                                                   6.3 × 10.sup.-7                                 ______________________________________                                    

where DC704 and DC705 are products of the Dow Corning Corporation: and

Convoil-20 is a product of the Sendix Corporation, and Santovac-5 is aproduct of The Monsanto Corporation.

The criteria for a resist 5 material that would facilitate practice ofthe invention are the rate of formation in an electron beam and thetolerance to ion milling when desired structure pattern is separatedfrom the layer 11.

TABLE 2 describes the resistance to ion milling properties of a 30nanometer thick Pd₄₀ Au₆₀ film at 5 kV and 0.5 ma/cm.

                  TABLE 2                                                         ______________________________________                                                            Electron dose (C/cm)                                      Oil     Oil deposited                                                                            at milling times of:                                       measured                                                                              (ml of Argon)                                                                            2.5 min   3 min   5 min                                    ______________________________________                                        Convoil-20                                                                            0.6        7.4 × 10.sup.-6                                                                   7.4 × 10.sup.-6                                                                 7.4 × 10.sup.-6                    DC704   5.2        8.7 × 10.sup.-7                                                                   8.7 × 10.sup.-7                                                                 7.7 × 10.sup.-6                    DC705   360        1.8 × 10.sup.-6                                                                   1.8 × 10.sup.-6                                                                 7.0 × 10.sup.-6                    Santovac-5                                                                            551        2.1 × 10.sup.-6                                                                   3.1 × 10.sup.-6                                                                 3.0 × 10.sup.-5                    ______________________________________                                    

From the tables it will be apparent that Dow Corning silicone oil typeDC705 is the best of the oils because it has a high rate of formation ofcontamination resist deposition in the electron beam and it will lastthe longest when subjected to ion beam etching. The DC705 oil istetraphenyl-tetramethyl-trisiloxane. The contamination resist mayexhibit low sensitivity and depletion effects when writing dense metalpatterns and hence a limitation is present on forming dense patterns forthese reasons. These effects can be offset by replenishing the oil inthe vapor or on the surface.

The resist pattern 6 will, as shown schematically in FIG. 4, exhibit ahigh aspect ratio in the vicinity of 3:1 and care should be used toprovide a uniform motion of the beam. The effect of a difference in timeunder the beam may be seen in connection with FIG. 7 wherein the conesthat are much higher than the lines are formed when the beam remains onone spot for an extended time.

Referring next to Step 2 in FIG. 6, since the goal is to produce apattern by using the electron beam 2 in the reaction zone at the pointof impact to grow the surface migrating resist 5 to a precise thicknessand width pattern 6, the scattering of the electrons are a measure ofthe dimensions. Control can be achieved by the electron beam 2 intensitybut since the electron beam 2 has several functions to perform inconnection with the conversion of the resist 5, the preferred method ofcontrol is the use of a monitor 9 with an aperture 14 that measureselectron scattering by the growing resist. Other methods of electronscattering monitoring in the light of the principles set forth will bereadily apparent including monitoring secondary electron scattering.

The resulting resist pattern may be used, as is, for such applicationsas electrical contact control and light modulation. If desired, forexample, in semiconductor applications directly on the crystal,separation of the pattern from the member 11 may be accomplished by ionor chemical etching wherein the resist pattern 6 serves as a delineatingmember to control the etching

The resulting 100 A range pattern may further then be used in situ or itmay, through the use of X-ray irradiation in the 20 A to 50 A wavelengthrange, be used as a mask in the transfer of the pattern to anothersubstrate for device formation.

The term "resist" has been used in accordance with the general use ofthe term in the art because in most applications the pattern is used forfurther etching delineation even though there are applications where thepattern is used directly.

What has been described is a technique of forming patterns of lines inthe sub-100 A range wherein the width and edge resolution are achievedby forming by electron beam conversion and fixing a width and thicknesscontrolled resist on a thin fill backscattering controlled layer whereinthe resist is the product of a monitored growth from a layer that isless than the thickness required for the desired pattern.

Having described the invention, what is claimed as new and what isdesired to secure by Letters Patent is:
 1. Apparatus for formingpatterns comprising in combination:an electron backscattering controlsupport having an electron backscattering control workpiece supportingsubstrate thereon, a controllable beam source of electrons focused onsaid substrate, electron beam monitoring means operable to sense achange in quantity of electrons passing through said substrate,communication means responsive to said monitor means and operable tocontrol said beam source; and aperture means operable to shield saidelectron beam monitoring means from electrons deflected by a formingresist.
 2. The apparatus of claim 1 wherein said backscattering controlsupport is one of the group of Si, Si₃ N₄, SiO₂, Al₂ O₃, polyimide,collodion and carbon.
 3. An apparatus for electron beam exposing aresist in the formation of a pattern, the improvement comprising patterndimension control comprising a monitor operable to control the electronbeam source in response to electrons passing through the member on whichthe resist is being formed and an apertured shield operable to preventelectrons deflected by the forming resist from reaching said monitor.